1. Field of the Invention
The present invention relates to a gate valve, and particularly to a gate valve having a sealing structure which exhibits excellent sealing properties for a long period.
2. Description of the Prior Art
Conventionally, as a sealing structure of a gate valve, known is a technique disclosed in Japanese Utility Model Application Laying-Open No. 4-39469. The gate valve is shown in FIG. 4.
The gate valve comprises: a pair of ring-like seal boxes 7, 7 which are fastened to the inner face of a valve body 1 by plural first fastening members 12, 12 each consisting of a bolt and a nut, so as to oppose each other; a pair of backup rings 10, 10 respectively fitted into faces of the ring-like seal boxes 7, 7 which oppose a sliding valve disc 2; and a pair of ring-like seal members 9, 9 which extend over faces of the backup rings 10, 10 which oppose the sliding valve disc 2, and also the opposing faces of the ring-like seal boxes 7, 7. The gate valve further comprises: a pair of ring-like valve body seats 5A, 5B which respectively oppose the backup rings 10, 10 with the respective ring-like seal members 9, 9 between, and which are placed so as to contact with the sliding valve disc 2; ring-like spacers 17, 17 which are adjacent to the inner sides of the ring-like valve body seats 5A, 5B in a radial direction, which oppose the ring-like seal boxes 7, 7 with the ring-like seal members 9, 9 between, and which are fastened to the seal boxes 7, 7 by plural second fastening members 15, 15 each consisting of a bolt; and ring-like retainers 14, 14 which are adjacent to the outer sides of the ring-like valve body seats 5A, 5B in a radial direction, which oppose the ring-like seal boxes 7, 7 with the ring-like seal members 9, 9 between, and which are fastened to the ring-like seal boxes 7, 7 by plural third fastening members 13, 13 each consisting of a bolt. The gate valve further comprises: a pair of seat rings 6, 6 which prevent the valve body seats from slipping out, which oppose the ring-like seal boxes 7, 7 with the ring-like spacers 17, 17 between, and which are placed in the vicinity of the both faces of the sliding valve disc 2; plural first springs 8, 8 which are interposed between the ring-like spacers 17, 17 and the seat rings 6, 6 so as to urge the seat rings 6, 6 toward the both faces of the sliding valve disc 2; plural second springs 11, 11 which are interposed between the ring-like seal boxes 7, 7 and the backup rings 10, 10 so as to urge the ring-like valve body seats 5A, 5B toward the sliding valve disc 2 through the backup rings 10, 10 and the ring-like seal members 9, 9; and a pair of ring-like gaskets 19, 19 which are interposed the mating faces 16 between the pair of ring-like seal boxes 7, 7 and the valve body 1.
In the sealing structure of the thus configured gate valve, the seat rings 6, 6 are slightly pressed against the sliding valve disc 2 by the urging of the first springs 8, 8. As the sliding valve disc 2 moves, therefore, foreign substances adhering to the both faces of the valve disc are scraped off by the seat rings 6, 6, so that the closeness between the ring-like valve body seats 5A, 5B and the sliding valve disc 2 is kept at a satisfactory state to maintain the sealing properties. Since the seat rings 6, 6 are pressed against the sliding valve disc 2 by the urging of the first springs 8, 8, the operation of scraping off foreign substances is continued even when the seat rings 6, 6 are abraded with the passage of time, so that the closeness between the ring-like valve body seats 5A, 5B and the sliding valve disc 2 is prevented from being reduced, thereby continuing the function of maintaining the sealing properties.
On the other hand, while being prevented from bending by the backup rings 10, 10, the ring-like valve body seats 5A, 5B are pressingly contacted with the both faces of the sliding valve disc 2 by the urging of the second springs 11, 11, thereby maintaining the sealing properties. Since the ring-like valve body seats 5A, 5B are pressed against by the urging of the second springs 11, 11, even when the ring-like valve body seats 5A, 5B are abraded with the passage of time, the pressing force exerted on the sliding valve disc 2 can be prevented from lowering. Consequently, the function of maintaining the sealing properties is not impaired.
Even when the sliding valve disc 2 is slightly moved toward the downstream side (for example, the left side in FIG. 4) in which the pressure is lower by fluctuation of the pressure acting on the both faces of the sliding valve disc 2, the movement toward the downstream side is absorbed by the urging of the second springs 11, 11, i.e., the expansion of the second spring 11 of the right side and the contraction of the second spring 11 of the left side, so that the ring-like valve body seats 5A, 5B are pressingly contacted with the both faces of the sliding valve disc 2, thereby preventing the function of maintaining the sealing properties from being impaired.
The sealing function of the ring-like seal members 9, 9 prevents foreign substances from entering from a passage 18 into the spaces of housing the second springs 11, and a fluid from leaking into the space of the valve body 1. The sealing function of the ring-like gaskets 19, 19 prevents the leakage at the mating faces 16 between the ring-like seal boxes 7, 7 and the valve body 1 from occurring.
In the prior art gate valve, since the ring-like valve body seats 5A, 5B are strongly pressed against the sliding valve disc 2 by the elastic forces of the second springs 11, 11 and the fluid pressure acting on the sliding valve disc 2, a large frictional force is generated between the ring-like valve body seats 5A, 5B and the sliding valve disc 2. When the sliding valve disc 2 moves in either of the directions indicated by arrows, therefore, the ring-like valve body seats 5A, 5B are pulled in the movement direction to slightly moved in spite that the ring-like spacers 17, 17 are fastened to the ring-like seal boxes 7, 7 by the second fastening members 15, 15, and that the ring-like retainers 14, 14 are fastened to the ring-like seal boxes 7, 7 by the third fastening members 13, 13. This produces a phenomenon that the ring-like valve body seats 5A, 5B rub together the ring-like seal members 9, 9. As the frequency of the movement of the sliding valve disc 2 in the directions of the arrows becomes high, the ring-like seal members 9, 9 are abraded to impair the sealing properties, so that foreign substances enter from the passage 18 into the spaces of housing the second springs 11. This develops the possibilities that the pressing forces of the ring-like valve body seats 5A, 5B against the sliding valve disc 2 are lowered to impair the sealing properties of the gate valve, and that a fluid leaks into the space of the valve body 1 through the spaces of housing the second springs 11.
On the other hand, the slight movement of the ring-like valve body seats 5A, 5B which is caused by the movement of the sliding valve disc 2 in the directions of the arrows is transmitted to the ring-like retainers 14, 14 and the ring-like spacers 17, 17, and further transmitted to the ring-like seal boxes 7, 7 through the second fastening members 15, 15 and the third fastening members 13, 13. This may cause the ring-like seal boxes 7, 7 to slightly move, whereby producing the possibility that the ring-like gaskets 19, 19 interposed at the mating faces 16 between the ring-like seal boxes 7, 7 and the valve body 1 are abraded so that the sealing properties are impaired, thereby causing a leakage from the mating faces 16 to occur.